Photothermography is a technique of forming images by imagewise exposing a light-sensitive material and uniformly heating the exposed material to develop the same, having the advantage of providing images by simple dry processing. As light-sensitive materials for the photothermography, those which use organic silver salts are well known.
On the other hand, many image-recording processes utilizing a photopolymerization reaction are known and they have been applied to form photoresists and printing plates. However, except for special cases (for example, using monomers which themselves have color-forming ability such as N-vinylcarbazole-carbon tetrabromide type light-sensitive material), it has been difficult to form a visible image by photopolymerization using ordinary monomers, which has been one reason for the rather limited application of photopolymerization systems (and photopolymer systems) in ordinary photographic recording.
In order to obtain visible images, exposed areas or unexposed areas are selectively colored with a dye solution or a pigment powder based on the difference in permeability or adhesiveness therebetween, or previously colored light-sensitive layers are imagewise exposed and exposed areas are separately spaced unexposed areas by development such as liquid development or delamination development utilizing the difference in physical properties (e.g., solubility or adhesiveness) between hardened areas and unhardened areas. The above-described processings require complicated procedures, and more convenient processes for obtaining visible images have been desired.
Several processes for forming visible images by dry processing directly utilizing a chemical reaction in a light-sensitive material, i.e., without subsequently coloring from the outside or separately spacing exposed areas from unexposed areas using a photopolymerizable composition (or light-sensitive resin) have been proposed. Japanese Patent Application (OPI) No. 89,915/77 (The term "OPI" as used herein refers to a "published unexamined Japanese Patent Application") describes a process of forming a visible image by thermal development using a photopolymerizable composition (or a photopolymerizable light-sensitive resin) and a heat-sensitive, color-forming material. In this process, a material containing two components, i.e., two-heat-sensitive, color-forming substances separately dispersed on the inside and outside or on different sides of a photopolymerizable composition (or light-sensitive resin) are imagewise exposed and heated. Areas hardened by the exposure do not undergo coloring since the heat-sensitive substance does not migrate, whereas unexposed areas undergo coloring since the substance migrates. A positive-positive recording response is thus obtained.
Japanese Patent Application (OPI) Nos. 179,836/82, 197,538/82, 23,024/83, and 23,025/83 describe processes of forming visible images by applying pressure using a photopolymerizable composition and a heat-sensitive (pressure-sensitive) color-forming material. In these processes, a material containing one component of two-components type color-forming substance microencapsulated together with a photopolymerizable composition and the other component disposed on the outside of the microcapsules is imagewise exposed, and pressure is applied thereto. In this system, exposure-hardened capsules are not ruptured and thus no coloration takes place in exposed areas, whereas capsules in unexposed areas are ruptured by the pressure to bring the two components of the color-forming substance into contact with each other, thereby causing as reaction therebetween them to form color. Thus, a positive-positive recording response is obtained.
All of these light- and heat-sensitive or light- and pressure-sensitive, image-recording processes using photopolymerization reaction are based on imagewise controlling the coloration reaction by a difference in material migration diffusibility between polymers and monomers. However, the degree of hardening by photopolymerization, i.e., the "barrier" to material migration is not necessarily proportional to the polymerization degree and, when the polymerization degree reaches a certain level (gelation point), rapid gelation takes place, leading to the sudden appearance of "barrier" properties. Therefore, in the above-described image-recording processes, image density is made proportional to the exposure amount only with difficulty. Thus, such processes are poor in gradation reproducibility. In addition, it is generally difficult for the polymer produced by the photopolymerization to completely stop material migration therethrough and, in order to obtain sufficient "barrier" properties, an extremely high exposure amount is required, which involves the defects of increased fog, an insufficient signal/noise ratio (S/N ratio), and insufficient recording sensitivity.